Queue up multiple instances of a shell script

https://blog.skbali.com/2019/03/queue-up-multiple-instances-of-a-shell-script/
https://forums.bunsenlabs.org/viewtopic.php?pid=112265#p112265

… how to queue up multiple instances of a shell script so that only one instance is active at a time. The other instances will queue up and run as soon as an active script finishes.